A transport network (TN) is used to carry data signals between a Radio Base Station (RBS), such as a NodeB or an eNodeB in 3G Long-Term Evolution (LTE) networks, and a Serving gateway (S-GW) or Packet Data Network gateway (PDN-GW). With the rapid growth of digital data telecommunications following the introduction of 3G and 4G technology, TNs, or more specifically nodes within TNs, may become the bottlenecks in the overall data transport process. This presents a problem for operators as a Service Level Agreement (SLA) between two operators (the TN operator and the mobile network operator) will likely specify a guaranteed minimum quality of service for the TN. Thus, various systems and methods have been proposed for improving or prioritising the way that data packets are transported across the TN.
A favoured candidate for the TN architecture is the so-called Metro Ethernet, due to its reliability and the ready availability of network components and interfaces. In the Metro Ethernet Forum (MEF), a coloring concept is defined in MEF 23, Carrier Ethernet Class of Service—Phase 1. In this scheme, packets for transport across the TN are assigned a color (green, yellow, or red) according to whether they are conformant with the bandwidths as specified in a Service Level Agreement (SLA) established between two operators. In the case of the Metro Ethernet, minimum bandwidths may be specified in terms of a Committed Information Rate (CIR, or green rate) and an Excess Information Rate (EIR, or yellow rate, that is the maximum acceptable rate).
Identification of color can be used to indicate which service packets are deemed to be within or outside of the contract according to the bandwidth profile. Levels of compliance are green when fully compliant, yellow when sufficient compliance for transmission but without performance objectives, and red or discarded when not compliant with either. To take account of the coloring of packets, at a bottleneck point within a TN, a color aware active queue management process discards yellow packets in preference to green packets.
For packets arriving at in injection point to the TN (where the injection point has access to and enforces the SLA) and which do not already have a color assigned to them, the injection point will assign a color, green or yellow, to them according to the available bandwidth and the SLA. A “green bucket” is replenished with green tokens at the green rate (CIR). Similarly, a “yellow bucket” is replenished with yellow tokens at the yellow rate (EIR). A packet arriving at the injection point at a point in time when there are sufficient tokens present in the green bucket is colored green and sent over the TN (with an appropriate number of tokens being extracted from the green bucket). On the other hand, if at that point in time there are insufficient tokens in the green bucket but there are sufficient packets in the yellow bucket, the packet is colored yellow and sent over the TN (with an appropriate number of tokens being extracted from the yellow bucket). If there are insufficient tokens in both the green and yellow buckets, the packet is colored red and discarded.
In some cases, the mobile network operator may pre-color packets green or yellow. At the injection point into the TN, a color aware profiler checks whether sufficient capacity exists for the received green and yellow packets (by inspecting the green or yellow bucket as appropriate). If sufficient capacity is available, the packets are sent over the TN with their colors unchanged. If sufficient capacity is not available, green packets may be re-colored yellow or red, and yellow packets may be re-colored red. Packets that are re-colored as red are dropped by the profiler (an appropriate retransmission mechanism is typically employed to cause redelivery of the dropped packets).
It is noted that the IEEE 802.1ad standard introduced a Drop Eligibility bit in VLAN tagged Ethernet frames (Virtual Bridged Local Area Networks Amendment 4: Provider Bridges, IEEE 802.1ad). The DEI bit is mandatory in service VLAN tagged frames and optional for customer VLAN tagged frames. The DEI bit provides a possible option to mark packets as green or yellow.
Pre-coloring of packets by the mobile network operator allows that operator to easily verify conformance of the TN operator with the SLA. The coloring concept can also be used to improve per-service fairness. Clients (UEs) making use of the mobile network service will typically establish a bearer to some gateway node, e.g. RNC in the case of 3G and S-GW in the case of LTE, for each session that they are engaged in. These bearers will be transported across the TN. The mobile network operator may wish to apply different TN rates and qualities to different session types. For example, the mobile network operator may wish bearers associated with voice calls to be sent over the TN with a relatively high rate and quality, whilst it may wish to transport bearers associated with P2P services (e.g. file sharing) with a relatively low rate and quality. This is possible by allowing the mobile network operator to define a green rate for an individual bearer. Packets on this bearer are colored green up to that green rate. Above the green rate, packets are colored yellow. Thus, for example, the green rate on a voice call bearer may be set low relative to the green rate set for a P2P bearer. The color aware profiler at the TN injection point will tend to send green packets over the TN in preference to yellow packets. Of course, the sum of the green rates for all bearers should not exceed the green rate specified by the SLA, i.e. the CIR.
In some cases, the TN injection point may implement a so-called color blind profiler. Such a profiler takes no account of any pre-coloring of the packets, and merely colors packets according to the SLA. In the case where pre-coloring is intended to improve “fairness” between different bearers, the color blind profiler does not achieve this. In the case where the profiler is color aware profiler, it is typically not possible for yellow packets to be changed to green. In this case it is possible that yellow packets will be injected into the TN when there remains capacity (according to the SLA) for green packets. The TN network is permitted to drop the yellow packets. Thus, a method intended to improve fairness between bearers has the result of unnecessarily reducing bearer transmission rate and quality across the TN.